One conventional system of the type is a system that employs an electronic counter in which a time interval between two time points on an analog input signal is measured on the real time basis, using an analog comparator; but this prior art system has a defect such that it is difficult to measure the time interval in a case where the input signal has a complex waveform. In the device of the type having an A/D converter and a memory, such as a digitizer, measurements of time intervals are not performed by specifying a trigger level, a slope and so forth.
In such a device as a digitizer, a digital sampling oscilloscope, a digital storage oscilloscope, a transientscope, etc., an analog input signal is converted at certain equal time intervals by an A-D converter to a digital value train and the converted signal waveforms are stored in a memory. Not only in the abovesaid device but also in means for analyzing the signal waveform once converted into a digital value, the measurement of the time interval in the signal is also an important item of analysis along with a frequency analysis, such as Fourier transformation, or voltage measurement. There is a method of measuring the time interval in the analog input signal after the signal converted to the digital value train is recoverted by a D/A converter to the original analog signal. However, this method requires a D/A converter also for digital setting of a trigger level, adjustment of an analog processing circuit added and countermeasures against external disturbances such as noise, temperature and so forth; accordingly, this method is defective in that the arrangement used is complex. Further, the addition of an electric counter or the like to the abovesaid device also results in the drawback of a complicated arrangement.
It is also possible to read out and process successively a stored digital signal train by software processing under the control of a control unit such as a microprocessor, a minicomputer or the like; but this is time-consuming.